A playback path is a logical unit of a digital stream recorded on a recording medium and defined by playback path information also recorded on the recording medium. A playback apparatus executes playback of the digital stream in accordance with the playback path information. With this technique, simply by defining a plurality of pieces of playback path information, the same digital stream can be played back with variations. With this advantage, the technique is considered essential in creating multi-story movies.
Playback path information may be defined using addresses on the recording medium or time information. In the case of playback path information defined using time information, the time information needs to be converted into address information of a specific point on the stream. The conversion is carried out by converting the time information specifying the start point of playback path (In_time) to the address of an I picture. Here, the address derived from the same In_time differs depending on the GOP structure of the video stream. In the case where the video stream is an MPEG2-Video stream and the In_time corresponds to a Closed-GOP, the first I picture of the GOP shall be read. Thus, the address of the first I picture in the GOP to which In_time belongs needs to be derived as a result of the conversion.
On the other hand, an Open-GOP has reference to the previous GOP and is not independently decodable. In order to correctly start playback from an Open-GOP corresponding to In_time, the playback apparatus needs to start reading the video stream from the immediately preceding GOP to the GOP corresponding to the In_time. As described above, playback of an Open-GOP requires the playback apparatus to read an extra GOP before reading the Open-GOP.
The following patent literature discloses a data structure for facilitating playback of a playback path defined using time information.    Patent Literature 1: JP Patent Application Publication No. 2001-169247 (Application No. 2000-228656)